1. Field of the Invention
This invention relates to a method of manufacturing lead barium titanate (Ba,Pb)TiO.sub.3 semiconducting ceramics having a positive temperature coefficient characteristic of resistivity.
2. Description of the Prior Art
In the prior art, semiconducting ceramics having a positive temperature coefficient characteristic of resistivity are produced by adding a very small amount of a rare earth element, antimony or bismuth to a barium titanate (BaTiO.sub.3) composite to make the composite semiconducting and the semiconducting ceramics thus obtained are employed as a heat sensitive element, a current limiting element, etc. It is also known in the art that, by substituting one part of barium in the barium titanate (BaTiO.sub.3) with lead, the temperature range in which the barium titanate (BaTiO.sub.3) semiconducting ceramics exhibit the positive temperature coefficient characteristic of resistivity can be shifted higher than 120.degree. C.
However, firing at a high temperature above 1250.degree. C is necessary for making this kind of composite semiconducting, so that, in the fabrication of (Ba,Pb)TiO.sub.3 semiconducting ceramics, firing in the air leads to evaporation of lead changing the basic composition, and making it impossible to obtain semiconducting ceramics of stable and desired characteristics. When the amount of lead substituted for the barium is in excess of 10 atomic %, such evaporation of lead rapidly increases and, in addition, many pores are formed in the surfaces of the semiconducting ceramics. Hence, this conventional method is impractical. To avoid these defects, methods such as firing in a lead oxide gas atmosphere have been proposed. However, such conventional methods substantially lower the resistivity changing ratio of the positive temperature coefficient characteristic of resistivity of the semiconducting ceramics or introduce considerable variations in their electrical characteristics, so that they are not suitable for large scale manufacture. Further, even if the amount of lead substituted for the barium is less than 10 atomic %, a little evaporation of lead is unavoidable, which is the primary cause of the variations in the electrical characteristics of the semiconducting ceramics.
For preventing evaporation of lead, it has been proposed to carry out firing in the atmosphere of an inert gas, for example, N.sub.2. However, where a ceramic material composed of (Ba.sub.0.6 Pb.sub.0.4)TiO.sub.3 and mixed with a very small amount of Y.sub.2 O.sub.3 was fired in the atmosphere of N.sub.2 gas, the point of abnormal change of resistivity relative to temperature disappeared as shown in FIG. 3 preventing the desired objects from being attained.